Method of making electrical resistors



May 14, 1946. H. F. FRLJTH ZLAQQ QZM- METHOD OF MAKING ELECTRICAL RESISTORS Filed Feb. 15, 1945 Patented May 14, 1946 METHOD OF MAKING ELECTRICAL RESISTORS Hal Frederick Fruth; Skokie, Ill.

Application February 15, 1945,

1 Claims.

The present invention pertains to a novel method of making electrical resistors, being especially suited to the manufacture of resistors of high ohmic resistance load capacity, such as are used in various electronic circuits in radio and other fields. It will be understood by those skilled in the art, however, that resistors may be made in accordance with the present invention of widely different characteristics from those noted.

The general aim of the present invention is to provide a method of making electrical resistors which makes possible high volume, low cost production.

More particularly, it is an object of the invention to provide a method of making resistors in which a suitably ridged dielectric core is given an overall coating of metal or other adherent conductive material, and then a quantity of such articles subjected to'an action such as that herein disclosed so as to remove simultaneously from all of the articles the coating along the ridges, leaving a highly resistant strip of coating in the intervening crevices and of a shape and length determined by the latter, all without the tedium of grinding or cutting out the coating on the ridges oi individual cores.

Serial No. 511,952

and comparatively small Further objects and advantages of the invention will become apparent as the following description proceeds, taken in connection with the accompanying drawing in which:

Figure 1 is a perspective view, partially in sectlon, of an electrical resistor made in accordance with the present invention.

Fig. 2 is a plan view of an apparatus suitable for use in carrying out the present invention.

While the invention is susceptible of various modifications, alternatives and variations, I have described in some detail the preferred mode of carrying out my invention, but it is to be understood that I do not thereby intend to limit the invention to the specific process disclosed, but intend to cover all modifications and alternative methods falling within the spirit and scope of the invention as expressed in the appended claims.

In practicing the present invention, a core of dielectric or non-conductive material having a ridged surface contour is provided. It may, for example, be of the form of the core l0 shown in Fig. 1 which is generally tubular and has an exterior of screw thread shape, the thread ll constituting a helical ridge of V-shaped cross-section. Such a core is of the familiar form used in wire wound resistors in which a wire of high resistivity is wound about the core in the root of the groove defined between adjacent convolutions of the thread. In practicing my method, however, no wire is employed, but the final product has instead a helical strip of conductive coating covering the walls of the groove between the thread convolutions. By making this coating of minute thickness. the end-to-end 'or overall resistance of the strip is made high even though the coating consists of such a good conductor as a noble metal. By varying the width of the strip, the end-to-end or overall resistance of the unit may be correspondingly varied.

It is with the achievement of such a conduc tive strip of precisely controlled resistance, but through procedures adapted for economical large-scale production, that the present invention is concerned.

As an initial step, the core is given an overall coating of conductive material. Preferably that is done by cathodic deposition of a noble metal in accordance with the processes described and claimed in my copending applicatiomfSerial No. 534,037, filed May 4, 1944, since that affords great precision of coating thickness, great tenacity of adherence for the coating and of even almost microscopic thinness when desired, while still retaining continuity and uniformity of coverage. So far as the present invention is concerned, however, the coating may be applied in other ways as, for example, by vaporization or other methods of metalizing or the like.

The next step is to remove the coating from the tops of the ridges on the core, leaving a conductive strip only between the ridges. For example, in the case of the device shown in Fig. 1, the problem is to remove the coating from the tip portions of the thread H, leaving a helical conductive strip i2 between the convolutions of the thread. What was initially an overall coating is thus opened to leave, what is in the case of the device in Fig. 1, a helical strip l2 of high resistivity. ihat the ridges must be free oi all conductive material, even the most minute deposits, will be evident, for otherwise adjacent convolutions will be short-circuited or likely to short-circuit under high voltage. That the distance a down the flanks of the thread from which the coating is removed must be determined with extreme precision will also be perceived; for that factor determines the width of the remaining metal strip and hence the resistance of the strip per unit length. In fact, the dimensions involved, are so minute that tolerance determination by measurement is not feasible, measurement resistance being required,

of actual electrical I have discovered that so exacting an operation may be accomplished at low cost and at a high production rate by employing a procedure which at first blush seems so remote from precision work as to be out or the question. I have found, however, that such cores can in fact be "opened" in batches in a tumbling mill and, more important, that. the operation can be carried out to give a predictable product in terms of resistance for the unit and that the resistance or the resulting units can be varied nearly linearly in relation to the time of duration or the tumblin operation.

In carrying out such tumbling, the apparatus used. speed or operation, etc., are governed to some extent by the size and shape of the cores and character of the coating. The general effects of such variance will, accordingly, be indicated. By way 01 specific example, let it be assumed that cores like the core I of Fig. 1 are to be tumbled, that they are 2" long, diameter, have 40 threads to the inch, have an overall coating of platinum iridium alloy or platinum silver alloy applied by cathodic deposition, and that the cores are made of fired ceramic material such as steatite. In such case a tour-sided glass vessel, like the jars ll in Fig. 2, may be employed in tumbling, the same being approximately 4" by 8" and having a smoothly contoured interior, rounded at the junctures of its walls and free of pockets or sharp corners. For larger work pieces, an appropriately larger vessel may be employed, and so too with smaller work pieces a smaller vessel may be employed it desired.

In the tumbling vessel I4 is placed a charge consisting of a batch of overall coated cores l0 and a loose mass 0! large granules of hard, dense material. These granules may be abrasive in character, being made, for example, of Carborundum or Alundum, and in such case serve to cut the coating from the protruding ridge portions or thread tips of the cores. On the other hand, the granules may be smoothly rounded pieces or hard, dense material such as haystellite and a finely powdered abrasive material used in conjunction with them. In such case the function oi the granules is to rub the powdered abrasive over the ridges of the cores to cut away the coating from the tips of the ridges. In either case the granules must be large enough that they cannot enter between adjacent ridges on the cores and must be tree of projections that can enter between such ridges. Moreover, the granules should be heavy enough to apply a strong rubbing force to the portions 01' the cores which they contact as they are flung across the same. The

granules should. however, be much smaller than the cores, being, for example, about $4 in diameter.

In making up a charge for one of the Jars I it may be filled about three-quarters full or charge to leave ample room for erratic motion of the charge constituents during milling. The charge may be proportioned with, for example, hair or its volume made up of cores and half 01 granules. If abrasive powder is used it may constitute say live percent, by volume, of the charge.

I! a finely divided abrasive materialis used, it should consist of such extremely light particles that they are substantially ineflective to cut the conductive coating on the cores except when rubbed across the coating by some heavier body, that is to say, by one of the other cores or by one of the heavy granules. Such inability of the abrasive particles alone to cut the coating is important since, being finely divided, it will inevitably enter into the grooves between adjacent ridges or threads on the cores during the turn-- bling process and consequently should be such that it cannot cut that portion of the coating unless pressed into contact by some heavier body. Since the granules themselves are too large to enter the root of the grooves, the presence oi abrasive alone in the grooves is quite sate. Moreover, the abrasive should be non-conductive so that any particles which remain on the finished product will not tend to short-circuit the resistor. Finely divided aluminum oxide of, for example, 300 mesh has been found suitable for the purpose.

Having loaded vessels such as the jars M in Fig. 2 with a charge consisting of cores and granules, with or without finely divided abrasive, depending upon the character of the granules, and in the proportions noted above, the vessels are moved in a manner to impart what may be best termed a churning motion to their contents. This motion oi the components of the charge should be highly random in character so that the movement of each core is erratic. By way of contrast, avalanching of the charge is to be avoided or, in other words, comparatively 510W rolling or sliding of cores down over others, since the latter type of motion is most likely to cause meshing oi the threads of one core with another. Such actual meshing is to be avoided for that would cause the threads or one core to enter deeply into the grooves in the core with which it was meshed, thus cutting the conductive coating away from the roots oi the grooves in both or the meshed cores. By imparting an erratic or churning motion to the contents or the vessel, such undesirable action can, however, be effectively avoided. In general, a large motion or the cores relative to each other throughout the body of the charge is to be accomplished, as distinuished from surface motion relative to a more or less static charge body.

A variety of mechanisms may be employed for imparting the desired type or churning motion to the charge. An example is illustrated in Fig. 2. As there shown, two identical vessels N are utilized, each having a screw top i5 for insertion and removal of their charges. These vessels are removably locked in sets of clamping iiners l6 mounted on the ends of generally oppositely projecting shafts l I in position to cant each of the Jars or vessels at an angle of about 45 degrees. A suitable drive is provided for rotating the shafts l1, including, in this instance, aspeed reduction gearing (not shown) contained in a housing It and driven by an electric motor ll. As the shafts II are revolved by the motor ll, the jar II are thus rotated about their longitudinal axes and moved bodily through orbital paths of generally circular form.

The speed of rotation for the vessels ll must be such as to impart the desired type or churning motion to their contents. It the speed is too slow, the motion of the charges will not be sufliciently erratic, while it too high the charge will be held more or less stationarily against the wall oi the vessels by centrifugal force and insufficient motion result. with the size vessel and character of charge assumed above. a speed of approximately 35 R. P. M. is satisfactory.

The tumbling oi the charges as described above is continued for a predetermined interval of time selected in accordance with the resistance desired for each 01' the resistor units being made.

The length or this time interval can be determined from a calibration chart prepared by trial of the particular apparatus and materials selected. When such calibration chart has been made, it will be found that it constitutes an accurate prediction of results for subsequent like batches, the resistance of the units produced varying substantially linearly with time. When the tumbling for a selected period is completed, the charges are dumped out of the vessels H, the re sistors cleaned and each of them tested for overall resistance. The bulk of them will be found to be of substantially the resistance value predicted from the calibration chart. Those which still have too low a resistance may be subjected to further tumbling in order to remove still more of their coatings, while any that are too high may be salvaged by recoating and tumbling them again.

I claim as my invention:

1. The method of making electrical resistors which comprises applying a thin overall coating of electrically conductive material to dielectric cores having screw thread shape external surfaces, and removingthe coating only along the tips of the threads on such cores by subjecting a batch of such coated core to an erratic churning motion along with a loose mass of hard granules which are substantially smaller than the cores but too large to enter the spaces between the a jacent thread convolutions on the cores.

2. The method of making electrical resistors which comprises applying a thin overall coating of electrically conductive material to dielectric cores having screw thread shape external surfaces, and removing the coating only along the tips of the threads on such cores by subjecting a batch of such coated cores to an erratic churning motion along with a loose mass of hard granules of, abrasive material which are substantially smaller than the cores but too large to enter the spaces between the adjacent thread convolutions on the cores.

3. The method of making electrical resistors which comprises applying a thin overall coating of electrically conductive material to dielectric cores having screw threaded external surfaces, and removing the coating only along the tips of the threads on such cores by subjecting a batch of such coated cores to an erratic churning motion along with a finely divided non-conductive abrasive and hard granules of rounded contour which are substantially smaller than the cores but too large to enter the spaces between the adjacent thread convolutions on the cores.

4. The method of making electrical resistors which comprises applying a thin overall coating of electrically conductive material to a dielectric core of ridged surface contour, and abrasively removing the coating along only the tips of the ridges on such core by tumbling it in the presence of a loose mass of granules which are substantially smaller than the core but too large-to enter the spaces between the ridges on the core.

5. The method of making electrical resistors which comprises applying a thin overall coating of electrically conductive metal to dielectric cores having screw thread shaped external urfaces, and a'crasively removing the coating only along the tips of the threads on such cores by partially filling a vessel with a charge comprising substantially equal parts by volume of such cores and granules of hard, dense, abrasive material, such granules being substantially smaller than the cores but too large to enter the spaces between the adjacent thread convolutions on the cores, and rotating the vessel while moving it through a generally circular path to subject its contents to an erratic churning motion.

6. The method of making electrical resistors which include the step of removing the coating only along the tips of the ridges on a ridged dielectric core having a thin overall coating of metal by tumbling it in the presence of finely divided aluminum oxide together with hard granules of smoothly rounded contour which are substantially smaller than the core but too large to enter the spaces between the ridges on the core.

'7. The method of making electrical resistors which comprises coating a plurality of screw threaded dielectric cores with an overall thin coating of metal on each, tumbling erratically a batch of such cores along with a loose mass of granules which are substantially smaller than the cores but too large to enter into the roots of the grooves between adjacent thread convolutions on the cores, continuing such tumbling for a time interval determined in accordance with the overall resistance desired for the resistors, and after the expiration of such interval selecting the resistors by individual test of their resistances.

- HAL FREDERICK FRU'IH. 

